Massive earthquakes provide new insight into deep Earth

In the waning months of 2018, two of the mightiest deep
earthquakes ever recorded in human history rattled the Tonga-Fiji
region of the South Pacific.

In the first-ever study of these deep earthquakes -- generally
defined as any earthquake occurring 350 kilometers or more
below the Earth's surface -- a Florida State University-led
research team characterized these significant seismological
events, revealing new and surprising information about our
planet's mysterious, ever-changing interior.

The team's findings, published in the journal Geophysical
Research Letters, delineate the complex geological
processes responsible for the earthquakes and suggest that the
first powerful perturbation may have actually triggered the
second.

"We don't have these kind of large earthquakes too often," said
study author Wenyuan Fan, an earthquake seismologist in FSU's
Department of Earth, Ocean and Atmospheric Science. "These deep
earthquakes, especially larger earthquakes, aren't really
promoted by the ambient environment. So why is this happening?
It's a compelling question to ask."

While deep earthquakes are rarely felt on the Earth's surface,
studying these titanic events can help researchers better
understand the systems and structures of the inner Earth.

But the precise mechanisms of deep earthquakes have long been a
mystery to earthquake scientists. The extreme temperature and
pressure conditions of the deep Earth aren't suitable for the
kinds of mechanical processes typically responsible for
earthquakes -- namely the movement and sudden slippage of large
plates.

Instead, the extraordinary pressure holds things firmly in
place, and the soaring temperatures make rocky material behave
like chocolate -- moving around viscously instead of like ice
cubes as is seen in the shallow surface.

"We did not expect to have deep earthquakes," Fan said. "It
should not happen. But we do have observations of deep
earthquakes. So why? How? What kind of physical processes
operate under such conditions?"

Using advanced waveform analyses, Fan and his team found that
the first quake -- a behemoth clocking in at magnitude 8.2,
making it the second-largest deep earthquake ever recorded --
was the product of two distinct physical processes.

The earthquake, they found, began in one of the region's
seismically important slabs, a portion of one tectonic plate
subducted beneath another. Slab cores are cooler than their
seething hot surroundings, and therefore more amenable to
earthquake nucleation.

Once the earthquake began forming in the slab core, it
propagated out into its warmer and more ductile surroundings.
This outward propagation moved the earthquake from one
mechanical process to another.

"This is interesting because before Tonga was thought to
predominantly only have one type of mechanism, which is within
the cold slab core," Fan said. "But we're actually seeing that
multiple physical mechanisms are involved."

The dual mechanism propagation pattern present in the magnitude
8.2 earthquake wasn't wholly surprising to Fan and his team.
The process was reminiscent of a similarly deep, 7.6 magnitude
quake that shook the region in 1994. These recognizable
patterns were a promising sign.

"To see that something is predictable, like the repeated
patterns observed in the magnitude 8.2 earthquake, is very
satisfying," Fan said. "It brings up the hope that we do know
something about this system."

But the second earthquake, which occurred 18 days after the
first, was more of a puzzle. The magnitude 7.9 convulsion
struck in an area that previously experienced very little
seismic activity. The distinct physical mechanisms present in
the second quake shared more similarities with South American
deep earthquakes than with the massive quakes that rock the
South Pacific. And, puzzlingly for researchers, the magnitude
7.9 earthquake produced surprisingly few aftershocks relative
to its considerable size.

Somehow, Fan said, a large earthquake was triggered in a
previously aseismic region that then immediately returned to
normal.

It's this triggering process that most interests Fan going
forward. He said this earthquake "doublet" illustrates the
dynamic and interrelated nature of deep-Earth processes and the
urgent need to better understand how these complicated
processes operate.

"It's important that we address the question of how large
earthquakes trigger other large earthquakes that are not far
away," he said. "This is a good demonstration that there seem
to be physical processes involved that are still unknown. We've
gradually learned to identify the pattern, but not to a degree
where we know exactly how it works. I think this is important
to any kind of hazard forecast. It's more than an intellectual
interest. It's important for human society."

This study was funded by the Postdoctoral Scholar Program at
the Woods Hole Oceanographic Institution.

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